There are certain truths about new motherhood that are unassailable. Things that lodge themselves in your psyche as permanently as the butternut squash stain on your last halfway decent T-shirt. The bone-deep exhaustion. The uneasy combination of anxiety and boredom. The pressure to bring sexy back when it feels like someone has driven a combine harvester through your nethers. All of which this latest collaboration between writer Diablo Cody and director Jason Reitman nails with harrowing accuracy.

It’s not exactly new territory. But what makes Tully such a tragicomic triumph compared with the brittle perkiness of films like I Don’t Know How She Does It (2011) and the god-awful Motherhood (2009) is that the film is not afraid to mine some pretty dark thematic territory.

This is thanks largely to a towering performance from Charlize Theron as Marlo, mother of three, including a newborn. Theron has perfected the dead-eyed gaze of a woman who can’t quite work out where motherly love ends and Stockholm syndrome begins. Baby weight and cupcake panic are tag-teaming to smother any spark of life she once had. Then Marlo cracks, and calls the night nanny for whom her wealthy brother has paid as a gift.

Enter millennial Mary Poppins, Tully (Mackenzie Davis), an unflappable free spirit who effortlessly shoulders the burden of motherhood. Marlo’s connection with her nanny is sudden and profound: Tully is like a window into her own past self.

The wistful, sometimes melancholic tone of this rueful examination of parenthood doesn’t blunt the edges of Cody’s acutely perceptive writing. And it is perhaps no coincidence that Reitman, who seemed tonally unmoored with his last two films – Men, Women & Children and Labor Day – returns to the incisive form last exhibited with Young Adult, his previous collaboration with Cody and Theron.

Tully is emotionally complex, bleakly funny and only slightly depressing.